Modeling of the Thread Tapping Process: Chip Formation and Cutting Fluid Lubrication
 
 

Tengyun Cao | PhD | 1998

Internal thread tapping is one of the most demanding machining processes. The most commonly encountered problems in the tapping process are tool breakage and poor thread quality. Cutting fluids are usually used to improve the performance of the tapping process. However, the use of cutting fluids creates serious environmental and health problems. A better understanding of the tapping force system, as well as the roles and effects of cutting fluids in the tapping process, would benefit both the tap manufacturers and tap users in enhancing productivity and achieving environmentally-conscious manufacturing. This research investigates the chip formation and cutting fluid lubrication in the tapping process to gain the knowledge necessary for process troubleshooting and the exploration of dry tapping.

A mechanistic model for the prediction of tapping torque and axial force has been developed. The model accounts for the loads resulting from chip formation and tool/ workpiece friction in the tapping process. The chip formation force model is based on the instantaneous chip load in the tapping process and an experimentally calibrated cutting pressure submodel. The frictional load model is built on an elastic asperity interference submodel for tool flank/workpiece interfacial pressure prediction and on the experimentally measured frictional coefficients under various machining conditions. The chip formation force model and tool/workpiece frictional force model has been integrated to form a comprehensive force prediction model. The model is capable of predicting tapping torque and axial force resulting from chip formation and tool flank/workpiece friction under various machining conditions, including dry tapping and tapping with different cutting fluids.
Extensive tests on tapping torque and axial force measurements were conducted to verify the predictive model. Characteristics of the measured tapping loads were studied. It was found that the total tapping load consists of a base load and a chip packing load. The base load results from chip formation and tool/workpiece friction. The predicted tapping load was found to be in good agreement with measured based load. The chip packing load is the result of chip clogging in the flutes, and is random in nature. The chip packing load may be many times of the base load, depending on tap geometries and the machining parameters. Factors causing severe chip clogging and excessive torque leading to tap breakage were also studied.
 

Additionally, two experimental case studies have been conducted to investigate the effects of machining conditions, particularly the lubricating conditions on thread size accuracy and on thread surface quality. It was found that, although not the most significant factors, cutting fluids do affect the surface roughness. The effect of different cutting fluids on thread sizes is significant, and dry tapping does not necessarily produce the most variation in thread form elements as compared to tapping with different fluids.

If you have any comments or suggestions please e-mail jwsuther@mtu.edu.